I got a Mac book for Christmas

When I read the chipboard on our windows computers, the subject line turns purple on the posts that I've read.

When I read the chipboard on the mac, the subject line stays blue...so I can't easily tell what posts I've read.

Is there a setting I can adjust to show what posts I've read? (Remember...I'm a mac-newbie...if it's complicated, I'll just read the chipboard on the old computers!)

Thanks for any help...